The efficient use of memory is important in the design and operation of image processors. For example, consumer products such as television systems may use image processors including MPEG-2 signal processing. The MPEG (Motion Picture Experts Group) signal compression standard (ISO/IEC 13181-2, May 10, 1994) is a widely accepted image processing standard which is particularly attractive for use with satellite, cable and terrestrial broadcast systems employing high definition television (HDTV) processing among other forms of image processing. Products using high definition displays require 96 Mbits or more of memory to temporarily store MPEG decoded frames prior to display. An MPEG processor requires these frames for motion estimation and compensation to reconstruct accurate images for display.
Systems which reconstruct images from MPEG decoded picture elements (pixels or pels) typically employ Differential Pulse Coded Modulation (DPCM). In DPCM processing a processor generates a prediction which anticipates the next pixel value. A summation network subtracts the prediction from the actual pixel value resulting in a difference which anticipates the next pixel value. This difference, known as prediction error, is generally smaller than the original pixel or prediction values, and processing the difference rather than the original pixel value reduces system bandwidth requirements. Ang et al., "Video Compression Makes Big Gains," IEEE Spectrum, October 1991, describes an MPEG encoder and decoder.
Memory efficient image processors use less memory to store image frames by recoding (recompressing) the block data prior to storage. In the spatial domain, reducing the number of bits per pixel used to store the image frames adversely affects the picture quality if the pixels can not be accurately reconstructed to their original bit size. Artifacts may occur, especially in smooth areas of the image. Memory reduction image processors should accurately quantize and dequantize the MPEG decoded signal as efficiently and economically as possible.
The present inventors recognize the desirability of providing an efficient data reduction system employing minimal hardware and software which will minimize artifacts the system introduces into the reconstructed data. A system according to the present invention satisfies these objectives.